I am looking to add a sub to my boat over the winter and looking for some advice on the best way to do this. Currently I have 1 kenwood 600 watt 4 channel amp (not sure exactly what model) and that is hooked up to the 4 rear in boat speakers and two tower speakers which are the same as the in boats. (polk db651) Front speakers are just hooked straight up to the radio. Just talking with some friends I was given a couple of ideas but wanted to see what others thought. 1. Add a mono amp and a sub? 2. Take old amp out and add a 5 channel with the seperate sub channel? Which would be the best way to do this and any suggestions on good amp/sub combos?

I have the same boat. I'm running the stock interior speakers off the deck, 2 wetsounds 485's with a syn4, a 12" clarion sub with its own mono amp and a 10" bazooka with built in amp. I have never been a fan of running speakers/sub off the same amp for some reason.

If it's the factory Kenwood or Excelon 4-channel amplifier, I have never been happy with the thermal performance when heavily loaded down in a boat. Subwoofers and tower speakers constitute the most demanding loads, especially when bridged to a sub. So I would run the Kenwood amplifier highpass to the six in-boat speakers and it should be a permanent and non-compromising solution in that application. And it's always a good idea to get the bow speakers off the source unit.
As for the tower and subwoofer amplifier(s) you may or may not be able to do this in a single chassis amplifier. With no more than the power demands of Polk Audio dB651s on the tower, if you select a moderate mass subwoofer, you can run the tower and sub with ease with either a three, four, five or six-channel amplifier. But I would maintain the existing amplifier to better distribute the workload and keep all amplifiers running more conservatively. As an example, a 3-channel amplifier like a JL Audio XD500/3 will do it. It's fullrange Class D and very efficient. PasMag recently tested this amplifier at over 650 total watts RMS into a 2-ohm load. Or, you could go with a JL Audio XD700/5 so that you have the ability to upgrade to a pair of tower HLCDs in the future. It's a good idea to plan for the future. It costs a bit more upfront but saves you in the long run. You'll definitely want to select a subwoofer in conjunction with the amplifier. Wetsounds, Kicker, Alpine and a host of other amplifier brands offer many of the same Class D benefits.

I'd figure out if I was going to run a free air, sealed, or ported enclosure than look at how much room you have to play with. How loud are you wanting the sub? Depending on the amp you may need to invest in 2 distribution blocks, 0 or 4 gauge wire, and a proper battery bank. More info on what your wanting out of your system and these guys can give u some great advice and opinions.

I want to build a custom box under the drivers area for a sub. I really like how my in boats sound right now but it is definately lacking in bass. I just dont want to cheap out on it and regret it in the future. I have not yet fully decided if I want to do anything with tower speakers yet but if I did eventually it would probably just be like 2 hlcds so nothing too crazy there either.

Mike,
For the subwoofer you have to put the horse before the cart. First, rough design the enclosure with the available space in consideration of the floor hump, your tolerance for leg room and the necessary shape. The boat will determine the maximum sub enclosure displacement in a given location (you have chosen the best location) and the enclosure will dictate the ideal sub driver or at least the ideal sub driver size. Making a basic cardboard mock-up and converting that to an internal displacement is where the work is, although in some boats it's as simple as taking a few measurements. Past this first phase, matching an enclosure with a sub is a matter of going by the numbers.

If that's the case you should take some of David's advice.
Get them inboats all on the current amp = $0
Decide what sub ( JL12w3 slot ported, it will fit ) and towers (rev10's).. Hehe
Now you know what amp to purchase.. Get the new Sinister6 coming Soon... It will crush on the revs and JL12 to there peak performance..
Two amps total and your boat will be rediculous enough with only purchasing one amp and sub now... Then getting the tower upgrade later
If you want to downsize the suggested sub or towers just downsize the amp if need be to match the recommended wattage for the speakers you choose

Think of what you want to do first, then while taking everything ( now and future upgrades) into account, get a amp and or speakers that will do what you want, so you never have to replace it again. "Do it once" is a great quote that comes to mind....

David- I was talking with my brother about making a mockup with carboard to see what size box we can fit there? I know that every sub is different but is there an approximate size I should be looking for? Right by the drivers feet there is just the piece of wood with carpet. I am going to take that down, see how much room I actually have behind that and then decide if I need to move it forward at all (lose some leg room) or if it is fine as is to build a box behind it. I just dont want to have to go through all of that if there is enough room already. Your help is greatly appreciated.

If under the helm is pretty square and easy to build a box for, you can take length x width x height and that'll equal your net volume in cubic ft which will also give u a good idea of the max volume box you can have. You will still have to take off your sub displacement and port displacement off the gross to get your net volume. Reaudio.com has a decent box builder on their site. Hope that helps a little bit.

Mike,
The larger Supra hulls of that year have an angled floor hump which requires a more complex enclosure. The 21V may have a flat sole under the drivers helm and that makes for a simple enclosure design (just measurements and no mock-up). If as expected, a side-firing 12-inch sealed or side-firing 10-inch bass-reflex should be an easy build and fit. Either might allow you to use the existing cover facade. Either should be plenty of bass. Neither should cause a loss of leg room. A slight advantage would go to the bass-reflex 10-inch over the sealed 12-inch.
A side-firing design will allow you to benefit from the boost that comes from boundary loading. A little distance between the side-firing woofer/port and the hull is advantageous.

I just looked at it yesterday and took quick measurements and we can do a box 12 inches deep 20 wide and 18 high without moving forward and losing legroom. Could probably go bigger if need be. With that being said and possibly going to add a set of hlcds at some point what would be my best option for a sub? In this situation is is that jl 700x 5 the way to go for amp?

Mike,
The JL Audio XD700/5 would be an ideal amplifier.
A 12W3 in a bass-reflex enclosure needs just a bit more enclosure displacement than what you specified. But another half an inch here and there and you've got it. As stated before, it's imperative to sound quality that the port and woofer side of the enclosure has a good 6 to7-inches of clearance from the hull interior surface. Given the hull angle this seems likely. Another option is the 13W3 in a sealed enclosure which would be slightly smaller than your dimensions. The bass-reflex 12-inch would have about a 1.5 dB output advantage over the sealed 13.5". Not a big deal but the bass-reflex 12-inch would be slightly more responsive and easier to drive.

Tru,
You may. And, I can see how either a little more power or the strictly regulated power could be a benefit with the 12W3. The chip set used in the lowpass scetion of the XD500/3 and XD700/5 was tested by Pasmag doing nearly 400 watts into a 2-ohm load. So not bad but I agree a little more would still be nice with a 12-inch.
The thing is that he has six in-boat speakers and it would be nice to get them all on external amplification. The MHD900.5 is strictly regulated but doesn't have independent front and rear power supplies like the MHD600.4 for example (although the MHD900.5 does have independent high and lowpass power supplies). As a result a differential in the front and rear impedance load will re-bias the power supply voltage for all four highpass channels. And that would result in a power loss. So while the HD or MHD900.5 is considered by many to be the top 5-channel amplifier, including me, it is typically relegated to four 4-ohm highpass speakers.
Another higher power option could be a combination of JL Audio XD600/6 and XD600/1.

Oh, I got ya but, I thought we said do the 6 inboats on his current amp (Kenwood).?. Then we were looking for a amp to run a pair of towers and a sub.. A amp good enough to run his current DB 6.5" for now and when he upgrades to HLCD it can run them as well.. That is where I deduced a 900.5 instead of a 700.5. He will be running bridged.. We both know on that sub it can handle up to 600w, properly tuned so I figured the extra 100 would help in this case, prolly more as we know JL is generous on their spec's.. Regulated.. You know what I'm getting at.. I didn't know he wanted to run 6 inboats and a sub off the JL amp, my bad... 400w is a nice number as well for the 12w3

Tru,
You are absolutely correct as far as I'm concerned on all of the above. The thing to remember is that the 900.5 will deliver less power bridged into 4-ohms than it will running four independent 4-ohm speakers. So whether to the six in-boat speakers or to the bridged tower application it's reducing the power supply voltage and cutting back the output power. Now if you bridged into an 8-ohm load it will deliver full power just like it does running discrete stereo into four 4-ohm speakers. And that would be awesome on four surf speakers (two in series per side) like Icon8s for example, especially with the robust nature of the strict regulation.

Got me all confused now haha. Is the JL 700.5 still the best way to go? You guys know a lot more about this than I do. I just want to do it right the first time so I dont have to change anything out. I can say once I put a sub and possibly a set of HLCD's I will be happy with the setup and not looking to add anything extra.

Mike,
Yes, we did go off on a tangent. But the XD700/5 would be good for a 12W3, especially in a bass-reflex enclosure. JL rates the 12W3 from 150 to 500 watts and shows the sweet spot to be 300 watts and that is exactly what the XD700/5 is rated at although that sub section has been tested at about a third more into a 2-ohm load. The 12W3 has a thermal rating of 500 watts and we have stretched that to 600 watts in a boat. You are always going to have some latitude with power. This combination is great if you want a balanced system that drives the bass with ease within the boat interior. If you want to project bass away from the boat or be the host when tied up then you start considering more power and that usually translates to separate amplifier chassis.